1. Field of Endeavor
The present invention relates to sealing of gaps between relatively rotating machine components to control fluid leakage therethrough, and in particular, to an improved form of leaf seal.
2. Brief Description of the Related Art
It is common practice in rotating machinery, such as gas and steam turbines, to prevent excessive fluid leakage between relatively rotating components by providing them with various types of sealing arrangements. For example, the relatively rotating components may be a shaft rotating within static structure, such as a bearing housing, or a diaphragm that divides areas at different pressures within the turbine. Another example is a stage of shrouded compressor or turbine rotor blades that rotates within a surrounding compressor or turbine casing. Common types of seal used in such situations are labyrinth seals, fin seals and brush seals.
In recent years, so-called “leaf seals” have been the subject of research and development to replace other types of seals in certain situations, particularly where a rotating shaft penetrates a diaphragm. Referring to the part-sectional pictorial sketch of FIG. 1, a typical leaf seal 10 in such a situation is installed around a shaft 12 between a higher pressure region 14 and a lower pressure region 16, and includes an annular array 18 of thin, resiliently flexible metal leaves, the individual leaves 20 conveniently being of a generally rectangular shape and oriented so that they present their side edges 22 to the fluid leakage flow 24 through the annulus 18 of the seal. To protect the side edges 22 of the leaves 20 and restrict leakage flow 24 through the annulus 18, the upstream and downstream faces of the seal are covered by side cheeks 30, 32, respectively, of a housing 28. The leaves 20 are cantilevered, with their radially outer ends held encastré and slightly spaced apart from each other in pockets 34 of a spacer component 40 of the housing 28 and with their lengths extending from the housing 28 towards the shaft 12, so that their radially inner edges 36 are adjacent to, or touching, the shaft surface. In fact, the leaves project inwardly from the housing 28 in a direction that is offset from the radial direction in the direction of rotation of the shaft, the direction of rotation being shown by the arrow 38. In this way, the inherent resilience of the leaves 20 can be used to allow them to bend away from the shaft 12 when small radial excursions of the shaft (so-called “shaft whirling”, due to rotor imbalance or large fluctuations in torque loading) causes the shaft surface to come into interfering contact with the free ends of the leaves.
Plainly, the way in which the radially inner edges of the leaves 20 meet the surface of the shaft 12, or other type of rotating component, will have a large influence on the efficiency of the seal. One problem that affects sealing efficiency is excessive “blow-down” of the leaves onto the surface of the rotor. Blow-down is the tendency of the leaves to be blown against the rotor surface by aerodynamic forces generated by rotation of the rotor in the direction of arrow 38, and the pressure differential across the seal. A limited amount of blow-down is desirable to create a good seal between the free ends of the leaves and the rotor, but excessive blow-down forces result in premature wear of the leaves and/or the contacting surface of the rotor.